The present invention relates to a method for driving a self-scanning light-emitting element array, particularly to a method for driving a self-scanning light-emitting element array using three-terminal light-emitting thyristors.
A light-emitting element array in which a plurality of light-emitting elements are arrayed on the same substrate is utilized as a light source of a printer, in combination with a driver circuit. The inventors of the present invention have interested in a three-terminal light-emitting thyristor having a PNPN-structure as a component of the light-emitting element array, and have already filed several patent applications (for example, Japanese Patent Publication Nos. 1-238962, 2-14584, 2-92650, and 2-92651). These publications have disclosed that a self-scanning function for light-emitting elements may be implemented, and further have disclosed that such self-scanning light-emitting element array has a simple and compact structure for a light source of a printer, and has smaller arranging pitch of light-emitting elements.
The inventors have further provided a self-scanning light-emitting device having such structure that a transfer portion including a transfer element array is separated from a light-emitting portion including a light-emitting element array (see Japanese Patent Publication No. 2-263668).
Referring to FIG. 1, there is shown a self-scanning light-emitting element array chip 10 in which a transfer portion 10-1 is separated from a light-emitting portion 10-2, and a driver circuit 40 for driving the transfer portion and light-emitting portion, the self-scanning light-emitting element array chip being a type of two-phase (clock pulses Ø1 and Ø2) driving and diode coupling. The transfer portion 10-1 comprises transfer elements T1, T2, T3, . . . , diodes D, and load resistors R1, R2, R3, . . . . The light-emitting portion 10-2 comprises light-emitting elements L1, L2, L3, . . . The transfer element and light-emitting element are composed of a three-terminal light-emitting thyristor, respectively.
The transfer portion 10-1 further comprises a Ø1 line 11, a Ø2 line 12, and a power supply (VGK) line 14. The Ø1 line 11 is connected to a Ø1 terminal 21 through a current limiting resistor 31 provided within the chip 10, the Ø2 line 12 is connected to a Ø2 terminal 22 through a current limiting resistor 32 provided within the chip 10, and the VGKline 14 is connected to a VGKterminal 24. The gate of a transfer element T1 is connected to a start pulse (Øs) terminal 23 through a current limiting resistor 33.
The light-transmitting portion 10-2 comprises a write signal (ØI) line 15 which is connected to a ØI terminal 25.
The driver circuit 40 comprises four CMOS inverters 50-1, 50-2, 50-3 and 50-5 each consisting of a PMOS transistor (normally on) 51 and an NMOS transistor (normally off) 52. Each high level terminal of the CMOS converters is connected to a common power supply (+5V) line (or +5V power supply) 48.
The driver circuit 40 further comprises an input terminal 41 for Ø1, an input terminal 42 for Ø2, an input terminal 43 for ØS, and an input terminal 45 for ØI.
A current limiting resistor 35 is provided between the CMOS inverter 50-5 of the driver circuit 40 and the ØI terminal 25 of the light-emitting element array chip 10, and outside the chip 10.
The operating voltage of the transfer portion 10-1 of the self-scanning light-emitting array shown in FIG. 1 is needed to be at least 2VD (VDis a forward voltage of PN-junction in a light-emitting thyristor). VDis about 1.5V when the material for PNPN-structure is GaAs, so that the minimum operating voltage for the transfer portion becomes 3 volts. In practice, the self-scanning light-emitting element array is operated by a single power supply of about 5 volts in order not to be unstable in operation due to a parasitic resistance and a noise.
In the conventional self-scanning light-emitting array shown in FIG. 1, when a power supply voltage of 5 volts is used, the turn-on voltage of a light-emitting thyristor in the light-emitting portion 10-2 is substantially equal to the forward voltage VD (1.5 volts) of PN-junction. Therefore, a voltage drop of 3.5 (=5-1.5) volts is caused across the resistor 35 provided outside the chip. Assuming that a current through the light-emitting portion is 10 mA (an average value in time), a power consumed in the resistor 35 is 35 mW. On the other hand, a power consumed in the light-emitting portion is 15 mW. Therefore, when a plurality of self-scanning light-emitting element array chips, for example 60 chips are arrayed to form an optical writing head, the total power consumed in the head when one light-emitting element per chip is lighted up becomes 3W (=50 mWxc3x9760 chips). The heat generated by consumed power causes the temperature rising of the self-scanning light-emitting element array chip, resulting in the problem of the decrease of the luminous efficiency of light-emitting elements. In addition, the optical writing head is positioned in a narrow and bad exhaust head environment, so that the temperature in the printer rises to have an effect on the image formed by an electrophotographic printer.
As to the effect on the image due to the temperature rising of self-scanning light-emitting element array chips, the following causes are conceivable.
(1) When a pattern being light in color such as half-tone is printed just after printing a table including horizontal rules, a part of the pattern corresponding to the horizontal rules is missed in color, resulting in the degradation of an image quality. This is because the particular distribution in temperature is caused on the chip due to the printing of horizontal rules, and the luminous efficiency of the light-emitting elements contributing to the printing of horizontal rules is decreased.
(2) Whereas the temperature of the head at the start of printing is low, the temperature within the printer is gradually increased, so that the light output of the head is varied. This variation is large at the beginning of the printing, resulting in the problem.
(3) In the case of a self-scanning light-emitting element array having a structure such that adjacent light-emitting elements may be possible to be lighted up at the same time, the temperature rising is varied based on whether one light-emitting element is lighted up together with the other light-emitting element. As a result, there is a problem in that the photographic density is varied depending upon a pattern to be printed.
(4) The volume of a body through which the heat is dispersed at the light-emitting elements at the both ends of the chip is one-half of that at the center of the chip, resulting in a high heat resistance of said body. Therefore, the temperature rising of the light-emitting elements at the both ends of the chip becomes two times that at the center of the chip. As a result, there is a problem in that the light output at the both ends of the chip is decreased.
In order to resolve these problems, the technique has been proposed in which a uniform temperature distribution may be realized through a chip by causing the power consumption at the transfer portion when the light-emitting elements are not lighted up (see Japanese Patent Publication Nos. 8-264838 and 11-170596). According to this technique, the problem of an image degradation in the case (1) described above may be addressed, but the temperature rising of the head becomes larger because the same power as that when all the light-emitting elements are lighted up is consumed. The percentage of light-emitting elements lighted up is less than 20% in the case of conventional color printing, so that it is not effective to design an optical writing head under the assumption that all the light-emitting elements are always lighted up. Also, this technique may not address the problem of the variation of light output at the beginning of the printing in the case (2) described above.
While the technique has been proposed in which the printing is not carried out when the temperature variation is extreme at the beginning of printing (see Japanese Patent Publication Nos. 10-119349 and 10-235936), this technique addresses only the problems in the case (2) described above. Furthermore, the temperature variation of the head is not uniform depending upon the pattern of an image and may not be corrected. In addition, the technique has been proposed in which the light output of light-emitting elements is compensated based on the time duration of lighting up by monitoring the anode voltage of light-emitting elements being on-state (see Japanese Patent Publication No. 9-311664), but a complicated circuitry is required in this technique.
An object of the present invention therefore is to provide a method for driving a self-scanning light-emitting element array, in which a power consumed in a write signal limiting resistor provided outside a chip to suppress a temperature rising of an optical writing head.
Another object of the present invention is to provide a method for driving a self-scanning light-emitting element array, the temperature dependency of light output thereof is small, i.e. the temperature coefficient of light output may be reduced.
A first aspect of the present invention is a method for driving a self-scanning light-emitting element array including a self-scanning transfer element array having such a structure that a plurality of three-terminal transfer elements each having a control electrode for controlling threshold voltage or current are arranged, the control electrodes of the transfer elements neighbored to each other are connected via first electrical means, a power supply line is connected to the control electrodes via second electrical means, and clock lines are connected to one of two terminals other than the control electrodes of each of the transfer elements; and a light-emitting element array having such a structure that a plurality of three-terminal light-emitting elements each having a control electrode for controlling threshold voltage or current are arranged, the control electrodes of the light-emitting element are correspondingly connected to the control electrodes of the transfer elements, and a write signal line connected to one of two terminals other than the control electrode of each of the light-emitting elements is provided. This method comprises a step of causing the difference between Low-level and High-level of a pulse voltage for a write signal supplied to the write signal line through a current limiting resistor smaller than the difference between Low-level and High-level of a pulse voltage for transfer supplied to the clock lines, so as to decrease a power consumed by the current limiting resistor.
A second aspect of the present invention is a method for driving a self-scanning light-emitting element array including a self-scanning transfer element array having such a structure that a plurality of three-terminal transfer elements each having a gate electrode for controlling threshold voltage or current are arranged, the gate electrodes of the transfer elements neighbored to each other are connected via first electrical means, a power supply line is connected to the gate electrodes via second electrical means, and clock lines are connected to an anode of each of the transfer elements; and a light-emitting element array having such a structure that a plurality of three-terminal light-emitting elements each having a gate electrode for controlling threshold voltage or current are arranged, the gate electrodes of the light-emitting element are correspondingly connected to the gate electrodes of the transfer elements, and a write signal line for applying a current connected to an anode of each of the light-emitting elements is provided. This method comprises a step of determining a voltage of a constant voltage source by which the write signal line is driven via a current limiting resistor so that the temperature coefficient of a light output at a constant current is cancelled by the temperature coefficient of a current supplied to the light-emitting element.